An ultrasonic (US) flow meter for installation into a measuring tube is described in European Patent Application No. EP 0 477 418 A1. The ultrasonic (US) flow meter for installation into a measuring tube is used for air-mass measurement in a motor vehicle engine. The unit includes two ultrasonic transducers which operate in alternation as transmission transducers or receive transducers as well as a reflector system for creating a reflection path passing through the measuring tube for ultrasonic waves between the particular ultrasonic transmission transducer and the particular ultrasonic receive transducer. A base plate/mounting plate is provided for accommodating the first ultrasonic transducer and the second ultrasonic transducer as well as the reflector system. Furthermore, the base plate/mounting plate is used for sealingly mounting the unit over a mounting hole for the unit. The first ultrasonic transducer and the second ultrasonic transducer are preferably placed symmetrically inclined toward one another and the reflector system at a predetermined angle. The reflector system is supported at a distance from the base plate/mounting plate within the measuring tube using a supporting construction in such a way that the reflector system within the measuring tube together with the ultrasonic transducers forms the reflection path necessary for flow measurement. On its side facing away from the measuring tube, the base plate/mounting plate preferably supports an electronic control unit and measuring unit between the two ultrasonic transducers. The reflector system preferably contains a single reflector body for forming a V-shaped reflection path. In addition, it is possible to design the reflector system in such a way that a W-shaped reflection path includes the transducer-side inside wall of the measuring tube diametrically opposed to the reflector body or the measuring tube-side surface of the base plate/mounting plate and that an at least low-reflecting auxiliary body made, for example, of felt is situated on the reflector body preferably centered between the beam axes of the ultrasonic transducers for avoiding an interfering reflection of scattered rays of the ultrasonic transmission transducer during the measurement. The reflector system may contain two reflector bodies for forming the W-shaped reflection path including the transducer-side inside wall of the measuring tube diametrically opposed to the reflector plane or the measuring tube-side surface of the base plate/mounting plate.
The supporting construction is manufactured from a rod assembly, the supporting construction being made up of a dish-shaped sheet metal construction open in the flow direction and the reflector system being situated on the bottom of the dish-shaped sheet metal construction. The side walls of the dish-shaped sheet metal construction have recesses for weight reduction and/or for the purpose of a gas or liquid exchange between the inside and outside of the dish-shaped sheet metal construction. The side walls may be formed or oriented in such a way that they may be used as a guide surface for flow harmonization and for forming a relevant partial current. Preferably, the dish-shaped sheet metal construction is of a one-piece design.
In other ultrasonic flow meter units known from the related art, a system of two ultrasonic transducers P1 and P2 is customarily used, which send ultrasonic waves to one another. The ultrasonic waves pass from one to the other transducer through the flowing medium at propagation times t1 and t2. The difference between propagation times in the flow direction, propagation time t1, and against the flow, propagation time t2, represents a measure of the flow rate of the medium to be measured.